Frédéric Verdier

Electrically driven matter transport effects in PoP interconnections

The migration issue of Package-on-Package (PoP) is investigated in this article. The migration phenomenon can be amplified as the density of solder bumps increases, while the paths are very finer as well in PoP.

Three-dimensional FEM simulations of thermomechanical stresses in 1.55 μm Laser modules

Abstract The purpose of this study is to present three-dimensional simulations using finite element method (FEM) of thermomechanical stresses and strains in 1550 nm Laser modules induced by Nd:YAG crystal Laser welds and thermal cycles on two main sub-assemblies: Laser submount and pigtail. Non-linear FEM computations, taking into account of experimental σ(e) measured curves, show that Laser welding process can induce high level of strains in columns of the Laser platform, bearing the Laser diode, responsible of an optical axis shift and a gradual drop of the optical power in relation with relaxation of accumulated stresses in the sub-assembly. In the case of thermal cycles, stresses can occur on elements sensitive to coefficient of thermal expansion mismatches such as solder joint between the Laser platform and thermoelectric cooler and as fiber glued into the pigtail leading to crack propagation with sudden drop of optical power. The main objective of the paper is to evaluate thermomechanical sensitivity and critical zones of the Laser module in order to improve mechanical stability after Laser weld and reach qualification standards requirements without failures. Experimental analyses were also conducted to correlate simulation results and monitor the output optical power of Laser modules after 500 thermal cycles (−40 °C/+85 °C VRT).

Low frequency gate noise in a diode-connected MESFET: measurements and modeling

The low frequency Schottky diode noise has been investigated in GaAs power MESFETs. For those devices, gate noise spectra are generally composed of 1/f and shot noise contributions. We have observed an increase by two orders of magnitude of the noise level when MESFETs are submitted to rf life-test. The increase of the 1/f noise can be explained by a modification of the gate space charge region extension. This interpretation is sustained by a reduction of the drain current transient magnitude and the inherent active trap density. A correlation is assumed between the increase of the shot noise level after rf life-test and a micro-plasma formation. Both 1/f noise and shot noise evolution might originate in a local increase of the electric field in the vicinity of the gate in drain access region. We have demonstrated that LF gate current noise is an early indicator of damage mechanisms occurring at the gate-semiconductor and passivation-semiconductor interfaces of the devices.

Simulations of Thermomechanical Stresses and Optical Misalignment in 1550-nm Transmitter Optoelectronic Modules Using FEM and Process Dispersions

Expertise of packaging for optoelectronic components requires the solution of optical, mechanical, and electrical problems in the same way. The purpose of this paper is to present three-dimensional simulations using finite-element method (FEM) of thermomechanical stresses and strains in 1550-nm laser modules induced by Nd:YAG crystal laser welds and thermal cycles on main subassembly laser submount. Nonlinear FEM computations, taking into account of experimental sigma(epsiv) measured curves, show that the laser welding process can induce high level of strains in columns of the laser platform, bearing the laser diode, responsible of an optical axis shift and a gradual drop of the optical power in relation with relaxation of accumulated stresses in the subassembly (W. M. Sherry et al., ldquoHigh performance optoelectronic packaging for 2.5 and 10 Gb/s laser modules,rdquo in Proc. Electron. Compon. Technol. Conf., 1996, pp. 620-627). Typical stresses are close to 160 MPa with drift about 5 MPa with the dispersion of energy level of the laser Nd: YAG beam. The introduction of both material and process dispersion in order to evaluate their impact on product lifetime distribution has been taking into account. In the case of thermal cycles, stresses can occur on elements sensitive to coefficient of thermal expansion mismatches such as solder joints between the laser platform and thermoelectric cooler and as fiber glued into the pigtail leading to crack propagation with sudden drop of optical power. A previous paper demonstrated that laser submount is the most sensitive part of optical system (Deshayes, et al., ldquoThree-dimensional FEM simulations of thermal mechanical stresses in 1.55 mum laser modules,rdquo Microelectron. Rel., vol. 43, no. 7, pp. 1125 -1136, Jul. 2003). Experimental analyses were also conducted to correlate simulation results and monitor the output optical power of laser modules after 500 thermal cycles ( -40degC/ + 85degC VRT).

A methodological approach for predictive reliability: Practical case studies

Abstract The paper first describes challenges that engineers face in predicting the reliability of microelectronic assemblies. In addition, the constraints such as temperature, temperature cycle or moisture may interact with each other. So it is not feasible to solve the reliability problems without a methodological approach. The paper presents also case studies to illustrate a methodology combining experiments and simulations.

Moisture induced effects in PoP

This paper presents a combined numerical and experimental methodology for predicting moisture-induced effects in Package-on-Package (PoP) assemblies. The hygroscopic warpage of the assembly “PoP” is measured. Its behavior is compared with the individual components “top” and “bottom”. The assembly “PoP” absorbs more moisture than the sum of the two individual components, while deforms less than either of the two individual components, which holds much more constrain inside the assembly. In addition, the hygroscopic warpage (20µm) adds to the thermal warpage (30µm) for the assembly “PoP”, which might lead to more serious reliability problem. Furthermore, a new experimental methodology for prediction of the moisture-induced warpage is validated, and simulation models match the experimental results.

Analytical model for thermally-induced warpage of POP

The thermally-induced deformations of package-on- package (POP) are investigated theoretically, numerically and experimentally in this paper. An analytical model of POP is newly proposed for prediction of the warpage of POP, which can be achieved by solving a few linear multiple polynomial equations rather than building a complex model in ANSYS, and leads to satisfying results. All the parameters needed for the analytical model are temperature-dependant material properties, and geometrical parameters. Measurements of the the warpage of POP under various temperature loads are made with a Profilometer Altisurf 500. Moreover, a 3-D linear finite element model was built. Both measurements and simulation validate our analytical model. The results from the experiments are presented and compared in detail with those from the 3-D linear finite element and theoretical analysis. The good agreements in the experimental and modelling results reveal the validation of 3-D linear finite element model in mechanics sense and the effectiveness of the numerical and theoretical models in physics predictions. The interest lies in a very easy use of this theoretical model to quickly obtain the warpage information for deciding whether the design is suitable and/or comparing the different POP circuits design.

Comparison of RF and DC life-test effects on GaAs power MESFETs

Abstract The reliability of ion-implanted power MESFETs submitted to RF and DC life-tests in impact ionization regime has been investigated by DC electrical characterization, LF noise analysis and Drain Current Transient analysis (DCTS). The two types of life-test have induced no degradation of the channel. RF life-test has induced DC parameter evolutions, but no significant drift of dynamic performances. Different evolutions of the interface state effective density in the gate-drain access region have been induced after DC and RF life-test.

Analysis of low frequency noise in GaN based HEMT technologies

In this paper, we present some results on low frequency noise of GaN HEMTs grown either on sapphire, SiC or Si. The evolution of LF drain and gate current noise is analysed in ohmic and saturation regime. Devices on sapphire grown either by MOCVD or by MBE present α H value in ohmic regime as low as 10 -4 , whereas for devices grown by MOCVD on SiC, α H extends from 5x10 -4 to 10 -2 . Low frequency gate current noise and coherence function have been measured to discuss possible correlation between drain and gate current noise. Devices with high reverse gate current exhibit high gate current noise. The coherence function increases when the I g /I d ratio becomes higher than 10 -5 which results in increase of the drain current noise due to contribution of gate current fluctuations.

Empirical modeling of LF gate noise in GaAs DCFET in impact ionization regime

The evolution of the 1/f gate noise in GaAs DCFET has been analyzed in the impact ionization regime. As the drain bias Vd is raised, a steep increase of the 1/f gate current noise is observed in correlation with the triggering of the impact ionization mechanism. A novel and empirical model of the 1/f low frequency gate current noise S/sub ig/ measured in the impact ionization regime is proposed. The following relation fits it with an exponential law: S/sub ig/=E exp (-F/Vd) (1/f), which is similar to the well-known dependence of the impact ionization rate /spl alpha/ on the drain bias.